The present invention relates to testing arrangements in general, and more particularly to an arrangement for testing a flow meter.
There are already known various constructions of flow meters, among them such which are incorporated into a pipeline and include rotating measuring wheels or similar rotating measuring members. For testing or calibrating such flow meters, there is often used, in series with the flow meter, a measuring or reference cylinder-and-piston unit which includes a housing or cylinder, in the interior of which there is slidably received a floating measuring piston which is displaced by the fluid being measured and which determines a comparison volume displaced out of the cylinder-and-piston unit between a start position and a stop position of the piston, such comparison volume being supplied to the flow meter. Typically, a piston rod is rigidly connected the the floating measuring piston and extends to the exterior of the cylinder. Usually, the duration of a testing operation, which takes place between the start position and the stop position, is determined by a predetermined number of first electrical pulses which are issued by the flow meter being tested. Then, a start pulse originating at the flow meter and issued after the end of the initial running-in displacement of the measuring piston triggers the counting or summation of a train of second electrical pulses which are issued by a pulse generator that responds to the movement of the piston rod. This counting or summation is then discontinued at the end of the testing run by the last pulse issued by the flow meter. The comparison parison volume which results from the counting or summation of the second pulses of the second pulse train is then compared with the volume which results from the first pulses of the first pulse train.
A testing arrangement of this type is known, for instance, from the U.S. Pat. No. 3,492,856. In this arrangement, the outwardly extending piston rod of the measuring piston is connected by means of a cross-beam rigidly with a toothed rack that is guided in sliding guides for movement parallel to the piston rod. This toothed rack then transfers the movements of the piston rod through a plurality of transmission gears to a shaft which is connected with a pulse generator. This pulse generator issues, in dependence on the rotational angular position of the shaft, a large number of electrical pulses. What is disadvantageous in this known testing arrangement is that, due to the rigid connection of the toothed rack with the piston rod, the second pulse train is already commenced to be issued at the very beginning of the movement of the measuring piston, even though the measuring piston has not yet covered or been displaced by the initial or running-in distance which is needed for the quieting or smoothing of the flow prior to the start of the testing run proper.
This problem has already been recognized and, in order to take this initial running-in distance of the measuring piston into account, it was proposed in one known testing arrangement to incorporate a time-delay member into the electric circuitry. This time-delay member inhibits the summation or counting of the pulses of the second pulse train, commencing with the first pulse issued by the pulse generator, until the end of its time-delay period. Then, the first pulse issued by the flow meter after the expiration of such time-delay period triggers, in its capacity as a starting pulse, the summation or counting of the pulses of the second pulse train. Herein, the initial running-in distance of the measuring piston until the beginning of the testing run proper is determined by the time-delay member, so that different initial running-in distances occur at different flow velocities. As a result of this, the amount of the fluid in the comparison volume that is available for the testing run may be too small at high fluid flow velocities.
In these testing arrangements, which have become known under the name "Miniprover", the dimensions of the measuring cylinder-and-piston units have been held so small that the entire testing arrangement can be mounted on a normal motor vehicle trailer. Under these circumstances, the comparison volume displaced from the measuring cylinder-and-piston unit by the measuring piston between its start and stop positions is nowadays so small that the measuring wheels of the flow meter being tested conduct only a few revolutions during the testing run. Typically, the volume of the fluid displaced by the measuring piston in such "Miniprovers" lies between 8 and 200 liters, depending on the size of the testing equipment. So, for example, the comparison volume is indicated to be about two gallons, that is, about 8 liters, in the U.S. Pat. No. 3,492,856. This means that the unavoidable tolerances in the shapes of the gear teeth, or an elastic deformation in the bulky scanning construction consisting of the piston rod, cross-beam and toothed rack, or even a minute irregularity in the issuance of the flow meter signals, have already marked deleterious influence on the accuracy of the measurement, resulting in substantial measurement errors. Experience has shown that the known "Miniprovers" tend to too great a scatter of the measurement results, because of the only small number of the pulses issued by the flow meter during the test run, the measurement results differing in dependence on the construction of the flow meter being tested.